This application claims priority to an application entitled xe2x80x9cAntenna Device in Radio Communication Terminalxe2x80x9d, filed in the Japanese Patent Office on Nov. 13, 2000 and assigned Serial No. 2000-345449, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an antenna device in a radio communication terminal such as a portable telephone, personal handyphone system (PHS) or personal digital assistant (PDA), and more particularly to an antenna device having a simple power feeding configuration while being capable of achieving a reduction in specific absorption rate (SAR).
2. Description of the Related Art
As the use of portable communication terminals has increased, much attention has been paid to the effects (SAR) on the human body, in particular, the head, when it is exposed to electromagnetic waves radiated from portable communication terminals. In such a portable communication terminal, current concentrates on the antenna of the terminal. As a result, electromagnetic fields are concentrated about the head of the user to which a radiation source, for example, the feeding point of the antenna, is closely positioned. This results in an increase in SAR.
Antennas for portable telephones, such as an extendable monopole antenna or a fixed helical antenna, are known in the art and externally protrude from the case of the portable phone. However, such a monopole type antenna has drawbacks in that it has a poor design and is inconvenient to use. Furthermore, the monopole type antenna cannot effectively compensate for the problems of SAR. For this reason, an internal antenna, which is internally mounted in a phone body, has been preferably used.
As an example of such an internal antenna, antenna systems for portable telephones have been proposed. As shown in FIG. 2, this conventional antenna system uses a dipole antenna (balanced feeding type antenna), and an electromagnetic shield plate in order to achieve a reduction in SAR. Now, the conventional antenna system will be described in more detail with reference to FIGS. 2a and 2b. 
Referring to FIGS. 2a and 2b, a printed circuit board (PCB) 1 mounted to a phone body is illustrated. As shown in FIGS. 2a and 2b, a dipole antenna 2 is attached to an upper portion of the PCB 1 and connected to the PCB 1 via a power feeding circuit 3. A transmitting/receiving circuit 4 is arranged at a lower portion of the PCB 1. The power feeding circuit 3 and transmitting/receiving circuit 4 are connected together via a coaxial cable (semirigid cable) 5. A speaker is mounted to a surface of the PCB 1 opposite to the PCB surface to which the dipole antenna 2 is attached.
The operation of the antenna system having the above mentioned configuration will now be described. Transmission power from the transmitting/receiving circuit 4 is fed to the power feeding circuit 3 via the coaxial cable 5. Thereafter, the transmission mode of the transmission power is passed to an unbalance/balance-transformer (a balun). In this state, the transmission power is fed to the dipole antenna 2. For simplicity, the balun is not shown in the Figures.
When the user uses the portable telephone, his head is positioned close to the speaker 6 mounted to the surface of the PCB 1 opposite to the PCB surface to which the dipole antenna 2 is attached. In this case, the PCB 1, internally mounted in the portable telephone, serves as an electromagnetic shield plate, thereby reducing the intensity of near electromagnetic fields acting around the head. Accordingly, it is possible to achieve a reduction in SAR due to the shielding effect of the PCB.
However, the above mentioned antenna system of FIGS. 2a and 2b, which uses the dipole antenna 2 as an internal antenna and relies on the PCB 1 as a shield plate, has the following problems:
First, the power feeding circuit 3 requires a power feeding line in order to feed power to the dipole antenna 2. For this reason, the manufacturing costs of the portable telephone are increased.
Second, where the power feeding line is used, it is necessary to mechanically and electrically connect the dipole antenna 2 to the PCB 1. For this reason, the manufacturing time required to assemble the elements of the portable telephone is increased. This also results in an increase in the manufacturing costs of the portable telephone.
Third, a coupling mechanism having a contact pin structure provided with electrical contacts may be used to simplify the assembling process of the portable telephone. However, the reliability of this coupling mechanism tends to degrade because it is always in its electrically turned on state.
Therefore, an object of the present invention is to provide an antenna device in a radio communication terminal, which is capable of feeding power to an internal dipole antenna while using a PCB of the terminal as a shield plate.
Another object of the invention is to provide an antenna device in a radio communication terminal which is capable of achieving a reduction in SAR in case by using a non-protruding dipole antenna and a PCB of the terminal as a shield plate.
In accordance with the present invention, these objects are accomplished by providing an antenna device in a radio communication terminal adapted to transmit and receive radio signals comprising: a primary radiation element arranged on a first surface of a printed circuit board included in the terminal; a power feeding circuit for feeding transmission power to the primary radiation element; and a dipole antenna arranged to face the primary radiation element while being spaced apart from the primary radiation element to define a capacitance and magnetic coupling space. The primary radiation element on the first surface may be arranged opposite to a second surface of the printed circuit board to which a speaker included in the terminal is mounted. The dipole antenna facing the primary radiation element may be mounted to an inner surface of a case included in the terminal. The inner surface of the case is positioned to face the primary radiation element. Preferably, the printed circuit board and primary radiation element have a high dielectric constant.